Relational & Logical Operators, if and switch Statements

Transcription

1 1 Relational & Logical Operators, if and switch Statements Topics Relational Operators and Expressions The if Statement The if- Statement Nesting of if- Statements switch Logical Operators and Expressions Truth Tables 2 Relational Operators < less than > greater than <= less than or equal to >= greater than or equal to == is equal to!= is not equal to Relational expressions evaluate to true or false. All of these operators are called binary operators because they take two expressions as operands. Practice with Relational Expressions var a = 1, b = 2, c = 3 ; Expression true/false Expression true/false a < c a + b >= c b <= c a + b == c c <= a a!= b a > b a + b!= c b >= c 3 4 Arithmetic Expressions: True or False Practice with Arithmetic Expressions Arithmetic expressions evaluate to numeric values. An arithmetic expression that has a value of zero is false. An arithmetic expression that has a value other than zero is true. 5 var a = 1, b = 2, c = 3 ; var x = 3.33, y = 6.66 ; Expression Numeric Value True/False a + b b - 2 * a c - b - a c - a y - x y - 2 * x 6

2 Review: Structured Programming All programs can be written in terms of only three control structures The sequence structure Unless otherwise directed, the statements are executed in the order in which they are written. The selection structure Used to choose among alternative courses of action. The repetition structure Allows an action to be repeated while some condition remains true. Selection: the if statement if( condition ) // body of if statement The braces are not required if the body contains only a single statement. However, they are a good idea and are required by the 104 C Coding Standards. 7 8 Examples Alert Screenshot if(age >= 18) alert("go Vote!"); if(value == 0) alert("you entered zero."); 9 <script type="text/javascript"> <!-- var age = 18; if(age >= 18) alert("go Vote!"); //--> </script> 10 Good Programming Practice Always place braces around the body of an if statement. Advantages: Easier to read Will not forget to add the braces if you go back and add a second statement to the body Less likely to make a semantic error Indent the body of the if statement 2 to 3 spaces -- be consistent! Selection: the if- statement if( condition ) /* the if clause */ /* the clause */ Note that there is no condition for the

3 Example Another Example if(age >= 18) alert("go Vote!"); alert("maybe next time!"); if(value == 0) alert("you entered zero."); alert("value = " + value); Good Programming Practice Nesting of if- Statements Always place braces around the bodies of the if and clauses of an if- statement. Advantages: Easier to read Will not forget to add the braces if you go back and add a second statement to the clause Less likely to make a semantic error Indent the bodies of the if and clauses 2 to 3 spaces -- be consistent! 15 if(condition1) if(condition2)... /* more if clauses may be here */ /* the default case */ 16 Another Example Gotcha! = versus == if(value == 0) alert("you entered zero."); if(value < 0) alert(value + " is negative."); alert(value + " is positive."); 17 var a = 2; if(a = 1) /* semantic (logic) error! */ alert("a is one"); if(a == 2) alert("a is two"); alert("a is " + a); 18

4 Gotcha! = versus == The statement if (a = 1) is syntactically correct, so no error message will be produced. However, a semantic (logic) error will occur. An assignment expression has a value -- the value being assigned. In this case the value being assigned is 1, which is true. If the value being assigned was 0, then the expression would evaluate to 0, which is false. This is a VERY common error. So, if your if- structure always executes the same, look for this typographical error. Multiple Selection with if if (day == 0 ) alert ("Sunday") ; if (day == 1 ) alert ("Monday") ; if (day == 2) alert ("Tuesday") ; if (day == 3) alert ("Wednesday") ; (continued) if (day == 4) alert ("Thursday") ; if (day == 5) alert ("Friday") ; if (day == 6) alert ("Saturday") ; if ((day < 0) (day > 6)) alert("error - invalid day.") ; Multiple Selection with if- if (day == 0 ) alert ("Sunday") ; if (day == 1 ) alert ("Monday") ; if (day == 2) alert ("Tuesday") ; if (day == 3) alert ("Wednesday") ; if (day == 4) alert ("Thursday") ; if (day == 5) alert ("Friday") ; if (day == 6) alert ("Saturday") ; alert ("Error - invalid day.") ; This if- structure is more efficient than the corresponding if structure. Why? 21 The switch Multiple-Selection Structure switch ( expression ) case value1 : case value2 :... default: : 22 switch Example switch ( day ) case 0: alert ("Sunday") ; case 1: alert ("Monday") ; case 2: alert ("Tuesday") ; case 3: alert ("Wednesday") ; case 4: alert ("Thursday") ; case 5: alert ("Friday") ; case 6: alert ("Saturday") ; default: alert ("Error -- invalid day.") ; Is this structure more efficient than the equivalent nested if- structure? 23 switch Statement Details The last statement of each case in the switch should almost always be a break. The break causes program control to jump to the closing brace of the switch structure. Without the break, the code flows into the next case. This is almost never what you want. A switch statement will work without a default case, but always consider using one. 24

5 Good Programming Practices Include a default case to catch invalid data. Inform the user of the type of error that has occurred (e.g., "Error - invalid day."). If appropriate, display the invalid value. If appropriate, terminate program execution (discussed in CMSC 201). Why Use a switch Statement? A switch statement can be more efficient than an if-. A switch statement may also be easier to read. Also, it is easier to add new cases to a switch statement than to a nested if- structure Logical Operators So far we have seen only simple conditions. if ( count > 10 )... Sometimes we need to test multiple conditions in order to make a decision. Logical operators are used for combining simple conditions to make complex conditions. && is AND if (x > 5 && y < 6) Example Use of && if(age < 1 && gender == "f") alert ("You have a baby girl!"); is OR if (z == 0 x > 10)! is NOT if (!(bob > 42)) Truth Table for && Example Use of Expression 1 Expression 2 Expression 1 && Expression nonzero 0 nonzero 0 0 nonzero nonzero 1 if(grade == "D" grade == "F") alert ("See you next semester!"); Exp 1 && Exp 2 && && Exp n will evaluate to 1 (true) only if ALL subconditions are true

6 Truth Table for Example Use of! Expression 1 Expression 2 Expression 1 Expression nonzero 1 nonzero 0 1 nonzero nonzero 1 Exp 1 && Exp 2 && && Exp n will evaluate to 1 (true) if only ONE subcondition is true. if(!(age >= 18)) /*same as (age < 18)*/ alert("sorry, you can t vote."); alert("you can vote."); Truth Table for! Operator Precedence and Associativity Expression! Expression Precedence Associativity 0 1 nonzero 0 33 ( ) left to right/inside-out * / % left to right + (addition) - (subtraction) left to right < <= > >= left to right ==!= left to right && left to right left to right = right to left 34 Some Practice Expressions More Practice var a = 1, b = 0, c = 7; Expression a b a + b a && b a b!c!!c a &&!b a < b && b < c a > b && b < c a >= b b > c True/False 35 Given var a = 3, b = 7, c = 21 ; evaluate each expression as true or false. 1. c / b == 2 2. c % b <= a % b 3. b + c / a!= c - a 4. (b < c) && (c == 7) 5. (c b == 0) (b = 5) 36